Self-assembly of inner skin hollow fiber polyelectrolyte multilayer membranes by a dynamic negative pressure layer-by-layer technique

In the past studies, electrostatic layer-by-layer (LbL) adsorption of oppositely charged polyelectrolytes has proven to be a promising method for the preparation of polyelectrolyte multilayer membranes (PEMMs). Till now, this method was mainly used to assemble flat sheet and tubular membranes. Since hollow fiber membrane has some advantages such as high-packing density, self-contained mechanical support and hence the consequent economical superiority, this study therefore seeked to assemble inner skin hollow fiber PEMMs by using a dynamic LbL adsorption technique. The assembly process was successfully accomplished by alternatively dynamically filtrating polyacrylic acid (PAA) and polyethyleneimine (PEI) on a hydrolyzed hollow fiber polyacrylonitrile (PAN) membrane under a negative pressure condition. In the case of pervaporation separation of 95 wt.% ethanol–water mixture (50 °C), the membrane obtained with only 4.5 and 6.5 bilayers had separation factor of 245 and 1338 while the permeate fluxes were 290 and 120 g/(m² h), respectively. The pervaporation separation behavior of various alcohol/water mixtures with the alcohols being t-butanol, 2-propanol and ethanol were also investigated. Finally, scanning electron microscopy and atomic force microscopy clearly confirms a uniform and defect-free layer formed on the inner surface of hollow fiber support. Since different polyelectrolyte pairs could be used to assemble PEMMs for different uses, it was expected that the dynamic negative pressure LbL adsorption technique could also potentially be used to prepare many types of PEMMs in other fields.     

氧化石墨烯/聚电解质层层自组装制备单价离子选择性膜

采用层层自组装法在改性聚丙烯腈(PAN)膜表面交替沉积聚乙烯亚胺(PEI)和聚丙烯酸-氧化石墨烯(PAA-GO)混合液,制得了单价离子选择性复合膜。X射线衍射(XRD)测试结果表明成功合成了氧化石墨烯(GO)并在复合膜中均匀分散。扫描电镜(SEM)观察结果证实了多层聚电解质PEI/PAA-GO成功地组装在基膜上,并用紫外-可见(UV-Vis)光谱进一步证实了组装过程的均匀性和连续性。接触角和性能测试表明加入GO后,复合膜的亲水性和单价阳离子的选择性明显增大。这种高通量、高选择性的防污复合膜在分离和水的软化方面有很好的应用前景。     

Preparation of polyelectrolyte multilayer membranes by dynamic layer-by-layer process for pervaporation separation of alcohol/water mixtures

In the past decades, the layer-by-layer (LBL) adsorption of oppositely charged polyelectrolytes has proven to be a promising method for the preparation of polyelectrolyte multilayer membranes. However, to obtain a good separation capability, LBL adsorption involved relatively long periods because 50–60 bilayers were normally required. The aim of this study was to develop such a new method that would allow simplification of the LBL procedure. LBL adsorption was proposed to proceed under a dynamic condition to prepare polyelectrolyte multilayer membranes. The polyacrylic acid (PAA) and polyethyleneimine (PEI) were alternatively deposited on polyethersulfone (PES) ultrafiltration support membrane under a pressure of 0.1 MPa. The polyelectrolyte multilayer membranes prepared by dynamic LBL process were compared with those prepared by the static LBL process for the pervaporation separation of water–ethanol mixture. The results suggested that a relatively high separation factor could be obtained with only four composite bilayers by using dynamic LBL process. The preparative conditions including bilayer number, filtration time of the first PAA layer, reaction time, ratio between polayanion and polycation concentrations, PAA molecular weight and salt addition were investigated. The pervaporation conditions such as feed temperature and water concentration in the feed were also evaluated. Under the temperature of 40 °C, the separation factor and the permeate flux of the polyelectrolyte multilayer membranes were about 1207 and 140 g/(m² h), respectively.     

Polyelectrolyte layer-by-layer self-assembly enhanced by electric field and their multilayer membranes for separating isopropanol–water mixtures

An electric field enhanced method is developed for fabricating layer-by-layer (LbL) self-assembly polyelectrolyte multilayer membranes. Three kinds of electric field enhanced polyelectrolyte multilayer membranes (EPEMs), poly(diallyl dimethylammonium chloride)/poly(styrenesulfonate sodium salt) (PDDA/PSS), poly(diallyldimethylammonium chloride)/poly(acrylic acid sodium salt) (PDDA/PAA) and polyethylenimine/poly(acrylic acid sodium salt) (PEI/PAA), were self-assembled on a reverse osmosis membrane (ROM). The pervaporation performances of EPEMs for separating isopropanol–water mixtures (90/10, w/w) are all superior to those of corresponding normal self-assembled polyelectrolytes membranes (PEMs), and the selectivity increases with PDDA/PSS, PDDA/PAA and PEI/PAA in order. For (PEI/PAA)₄PEI EPEM, the separation factor is 1075 and permeation flux is 4.05 kg m⁻² h⁻¹ at 70 °C. This novel method speeds up the LbL process, which makes it promising for the practical application of the LbL multilayer membrane.     

Ultrathin self-assembled polyelectrolyte membranes for pervaporation

Ethanol–water pervaporation through new composite membranes with ultrathin self-assembled polyelectrolyte separating layer is described. The composite membranes were prepared by alternating electrostatic adsorption of poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate sodium salt) (PSS) on a porous PAN/PET supporting membrane (a polyethylene terephthalate fleece coated with a thin layer of polyacrylonitrile). The sealing of the pores of the supporting membrane was studied by gas flow measurements. Pervaporation experiments were carried out under variation of the preparation and operation conditions. Generally it was found that the separation capability considerably increased, when the composite membrane was annealed at temperatures above 60°C, while the flux simultaneously decreased. The same was found, when the number of PAH/PSS layers was increased. Raising the pervaporation temperature led to both an increase of the flux and the separation factor. The highest separation factor of 70 was found at a low water content of the feed of 6.2% (w/w). The corresponding flux was 230 g m⁻² h⁻¹. Pervaporation was feasible up to a water content of 24% (w/w) in the feed. At higher values, hydrolysis set in resulting in partial desorption of the separating layer.     

Self-assembled polyelectrolyte multilayer membranes with highly improved pervaporation separation of ethanol/water mixtures

Ethanol/water pervaporation through ultrathin polyelectrolyte multilayer membranes is described. The membranes were prepared by the layer-by-layer technique, i.e. by alternating sequential adsorption of cationic and anionic polyelectrolytes on a porous support. The separation capability was optimized by variation of the chemical structure of the polyelectrolytes, by variation of pH and ionic strength of the polyelectrolyte solutions used for membrane preparation and by annealing of the polyelectrolyte membranes. It was found that the separation is mainly affected by the charge density of the polyelectrolytes which is controlled by the chemical structure and the degree of ionisation of the polar groups. Selectivity for water was highest, if polyelectrolytes of high charge density such as polyethyleneimine (PEI), polyvinylamine (PVA) and polyvinylsulfate (PVS) were used and if the pH of the polyelectrolyte solutions was equal to the mean of the pK_a values of the corresponding cationic and anionic polyelectrolyte. Best results were obtained for PVA/PVS and PEI/PVS membranes which are characterized in detail with regard to their separation behavior.     

聚电解质PDDA/PSS层层自组装膜的渗透汽化性能

采用聚电解质层层自组装(LbL)技术, 在不同盐浓度下制备了聚(二烯丙基二甲基氯化铵)/聚苯乙烯磺酸钠(PDDA/PSS) 多层自组装膜, 并用于渗透汽化性能的研究. 重点考察了组装溶液中NaCl的浓度、组装层数及操作温度对自组装膜的异丙醇脱水性能的影响. 同时, 用扫描电镜观测了不同条件下制备膜的表面形貌. 结果表明, 在高NaCl含量的聚电解质溶液中只需组装几个双层的LbL膜, 即能获得较高的分离因子和较大的通量, 并解释了该LbL膜呈现反“trade-off”现象的原因.     

Chitosan/poly(tetrafluoroethylene) composite membranes using in pervaporation dehydration processes

Chitosan/PTFE composite membranes were prepared from casting a γ-(glycidyloxypropyl)trimethoxysilane (GPTMS)-containing chitosan solution on poly(styrene sulfuric acid) grafted expended poly(tetrafluoroethylene) film surface. The adhesion between the chitosan skin layer and the PTFE substrate was pretty good to warrant the high performance of chitosan/PTFE composite membranes using in pervaporation dehydration processes on isopropanol. The chitosan/PTFE membrane exhibited a permeation flux of 1730 g/m² h and a separation factor of 775 at 70 °C on pervaporation dehydration of a 70 wt% isopropanol aqueous solution. The membrane also survived after a long-term operation test in 45 days.     

Polyamide-imide/polyetherimide dual-layer hollow fiber membranes for pervaporation dehydration of C1–C4 alcohols

To circumvent the common swelling and deteriorated performance of integral asymmetric hollow fiber membranes for pervaporation dehydration, we have developed novel polyamide-imide (PAI)/polyetherimide (PEI) hollow fiber membranes with synergized performance with the aid of dual-layer spinning technology. Dehydration of C1–C4 alcohols has been conducted and the orders of their fluxes and permeances have been analyzed. The hollow fibers spun at 2 cm air gap and annealed at 75 °C exhibit the highest pervaporation performance: separation factors for t-butanol/water and iso-butanol/water binary systems are greater than 50,000 with flux more than 700 g/m² h. A comparison with literature data shows that the newly developed membranes outperform most other polymeric membranes for the dehydration of IPA and butanols. The dual-layer hollow fiber membranes also exhibit good long-term stability up to 200 h. The superior performance can be attributed to (1) the balanced properties of PAI as the selective layer for dehydration pervaporation; (2) the low water uptake and less swelling characteristic of the PEI supporting layer; and (3) the desirable membrane morphology consisting of a fully porous inner layer, a porous interface, and an ultrathin dense-selective outer skin.     

Effect of separating layer in pervaporation composite membrane for MTBE/MeOH separation

The composite membranes with polyvinylalcohol (PVA) as separating layer material and polyacrylonitrile (PAN) or cellulose acetate (CA) as supporting layer material were prepared for separating methyl tert-butyl ether (MTBE)/MeOH mixture by pervaporation (PV). The results showed that PV performance of the composite membrane with PVA membrane as separating layer was superior to that with CA membrane as separating layer, and the PV performance of PVA/CA composite membrane with CA membrane as supporting layer was better. The parameters to prepare the composite membrane remarkably affected PV performance of the composite membrane. The permeate flux of both composite membranes of PVA/PAN and PVA/CA was over 400 g/m² h, and the concentration of MeOH in the permeate reached over 99.9 wt.% for separating MTBE/MeOH mixture.     

PREPARATION OF PVA/PAN PERVAPORATION COMPOSITE MEMBRANES AND THEIR SEPARATION PROPERTIES

制备了活性层厚度为１～１０μｍ的ＰＶＡ／ＰＡＮ渗透汽化复合膜并将其用于乙醇水混合物的分离。实验结果表明，热处理条件对复合膜的分离选择性、渗透通量及分离指数具有明显影响。确定了最佳热处理条件。     

Composite membranes prepared from glutaraldehyde cross-linked sulfonated cardo polyetherketone and its blends for the dehydration of acetic acid by pervaporation

Cardo polyetherketone (PEK-C) composite membranes were prepared by casting glutaraldehyde (GA) cross-linked sulfonated cardo polyetherketone (SPEK-C) or silicotungstic acid (STA) filled SPEK-C and poly(vinyl alcohol) (PVA) blending onto a PEK-C substrate. The compatibility between the active layer and PEK-C substrate is improved by immersing the PEK-C substrate in a GA cross-linked sodium alginate (NaAlg) solution and using water–dimethyl sulfoxide (DMSO) as a co-solvent for preparing the STA-PVA-SPEK-C/GA active layer. The pervaporation (PV) dehydration of acetic acid shows that permeation flux decreased and separation factor increased with increasing GA content in the homogeneous membranes. The permeation flux achieved a minimum and the separation factor a maximum when the GA content increased to a certain amount. Thereafter the permeation flux increased and the separation factor decreased with further increasing the GA content. The PV performance of the composite membranes is superior to that of the homogeneous membranes when the feed water content is below 25 wt%. The permeation activation energy of the composite membranes is lower than that of the homogeneous membranes in the PV dehydration of 10 wt% water in acetic acid. The STA-PVA-SPEK-C-GA/PEK-C composite membrane using water–DMSO as co-solvent has an excellent separation performance with a flux of 592 g m⁻² h⁻¹ and a separation factor of 91.2 at a feed water content of 10 wt% at 50 °C.     

Pervaporation performance of crosslinked polydimethylsiloxane membranes for deep desulfurization of FCC gasoline: I. Effect of different sulfur species

Crosslinked PDMS/PEI composite membranes were prepared, in which asymmetric PEI membrane prepared with phase inversion method was acted as the microporous supporting layer in the flat-plate composite membrane. The different function composition of the PDMS/PEI composite membranes were characterized by reflection FTIR. The surface and section of PDMS/PEI composite membranes were investigated by scanning electron microscope (SEM). The infinite dilute activity and diffusion coefficients of thiophene, 2-methyl thiophene, 2,5-dimethyl thiophene, n-butyl mercaptan, n-butyl sulfide in crosslinked PDMS were measured in the temperature range of 80–100 °C by inverse gas chromatography. The solubility parameters of thiophene, 2-methyl thiophene, 2,5-dimethyl thiophene, n-butyl mercaptan, n-butyl sulfide were calculated by the group contribution method and the selectivity of PDMS composite membrane for different organic sulfur compounds was investigated. The composite membranes prepared in this work were employed in pervaporation separation of n-heptane and different sulfur forms mixtures. The theoretical results showed good agreement with the experimental results, and the order of partial permeate flux and selectivity for different organic sulfur compounds was: thiophene > 2-methylthiophene > 2,5-dimethylthiophene > n-butyl mercaptan > n-butyl sulfide, which should be significant for practical application.     

醇／水混合液分离用聚乙烯醇复合膜的渗透汽化特性

制备了聚乙烯醇（ＰＶＡ）／聚丙烯睛（ＰＡＮ）渗透汽化复合膜，研究了交联剂用量、底膜结构、进料液组成、操作温度等因素对膜的渗透汽化性能的影响．发现ＰＶＡ／ＰＡＮ复合膜对水／醇混合液表现为水优先透过，进料液中乙醇浓度在６０～９９ｗｔ％的范围内，渗透通量Ｊｔ与温度之间符合Ａｒｒｈｅｎｉｕｓ关系，选择分离系数αＷ／Ｅ也随温度上升而增大．进料液为９５ｗｔ％的乙醇／水混合液时，７５℃下Ｊｔ高达３００～４５０ｇ／ｍ２ｈ，αＷ／Ｅ为８００～１１００．对异丙醇／水、异丁醇／水及甘油／水混合体系，复合膜显示出更为优秀的透过、分离性能．就膜的化学、物理结构与其渗透汽化性能间的关系进行了讨论．     

Pervaporation separation of water/alcohol mixtures using composite membranes based on polyelectrolyte multilayer assemblies

Composite pervaporation membranes composed of an asymmetric polyamide-6 membrane and an ultrathin self-assembled polyelectrolyte separating layer are described. The supporting membrane was prepared from both an unmodified polyamide-6 and a comb-like polymer with carboxyl terminated polyamide-6 side chains. A high end group concentration was found to be advantageous for sufficient adhesion of the multilayer systems on the supports. Up to 20 layers were deposited onto the membrane surface by dipping the membranes in aqueous solutions containing oppositely charged polyelectrolytes. The polyanions used were poly(acrylic acid), poly(styrene sulfonic acid) and alginic acid. The polycations used were poly(diallyldimethylammoniumchloride), chitosan and poly(ethylenimine). Performance of these membranes depends strongly on the layer number and on the type of polyelectrolytes. In general, membranes modified with two weak polyelectrolytes of high charge density gave the best separation properties while those modified with strong polyelectrolytes of low charge density led to poorer separation properties. However, the highest separation factor (≥10,000) for a water/2-propanol mixture (12/88 w/w) at permeate flux of 300 g/m²h was obtained with six double layers consisting of poly(ethylenimine) and alginic acid. These composite membranes were stable over an operating period of at least 400 h.     

Self-assembled polyelectrolyte multilayered films on Nafion with lowered methanol cross-over for DMFC applications

The paper is concerned with the deposition of self-assembled polyelectrolyte multilayer on Nafion membrane by layer-by-layer (LbL) technique with lowered methanol cross-over for direct methanol fuel cell (DMFC) applications. The formation of self-assembled multilayered film on Nafion was characterized by UV–vis spectroscopy and it was found that the polyelectrolyte layers growth on the Nafion surface regularly. Furthermore, the proton conductivity and methanol cross-over measurements were carried out for characterization of the LbL self-assembled composite membranes. The results showed that the concentration and pH of the polyelectrolytes significantly affect the proton conductivity and methanol barrier properties of the composite membranes. 10⁻¹ monomol polyelectrolyte concentration and pH 1.8 was found to be optimum deposition conditions considering proton conductivity and methanol permeation properties of the LbL self-assembled composite membranes. The methanol permeability of the 10 bi-layers of PAH1.8/PSS1.8 deposited LbL self-assembly composite membrane was significantly suppressed and found to be 4.41 × 10⁻⁷ cm²/s while the proton conductivity value is in acceptable range for fuel cell applications.     

Pervaporation properties of novel alginate composite membranes for dehydration of organic solvents

A novel organic dehydration membrane consisting of aminated polyacrylontrile (PAN) microporous membrane as sublayer, alginate coating as top layer has been prepared and characterized by pervaporation experiment. The influence of hydrolysis and amination of the microporous support layer on selectivity and flux was studied and it was shown that amination of the sublayer improved pervaporation performance of the composite membrane greatly. The counter cation of alginate coatings as dense separating layer also influenced separation properties of the membrane, which was better for K⁺ than for Na⁺. This novel composite membrane with K⁺ as counter ion has a high separation factor of 1116 and a good permeation rate of 350 g/m² h for pervaporation of 90 wt.% ethanol aqueous solution at 70°C, higher separation factors and fluxes for n-PrOH/water, i-PrOH/water, acetone/water and dioxane/water systems. The results show that the separation factor and flux of this membrane increase with raising the operating temperature. At the same time, SEM micrographs show that the hydrolysis and amination of PAN microporous membrane change its pore structure. From the results it can be concluded that pore structure of the sublayer in addition to its chemical structure also make influence of separation properties of the composite membrane.     

渗透气化法分离液体混合物

渗透气化法是一种用来分离液体混合物的膜分离方法。本综述简要地讨论了渗透气化法的机理。本法用于乙醇-水混合物的分离,对于从生物源生产乙醇具有重要意义。乙醇-水混合物的分离膜可以分成二类:亲水性膜和亲乙醇膜。使用若干物理方法可改进渗透气化过程。最后,作者认为渗透气化膜的发展趋势是从均质膜,不对称膜向复合膜过渡。     

壳聚糖／聚丙烯酸聚电解质复合物膜对水／有机液体系的渗透汽化性能

以壳聚糖和聚丙烯酸为原料制备了聚电解质复合物膜，并对其分离水／乙醇体系的渗透汽化特性和浓度，温度，化学组成等因素的影响进行了研究，发现后处理方法对复合物膜的分离性能影响很大。同时对其它水／有机液体系，该膜也具备优异的分离性能。     

Pervaporation separation and mass transport of ethylbutanoate solution by polyether block amide (PEBA) membranes

Dense and composite membranes were developed from polyether block amide (PEBA). Polyacrylonitrile (PAN) and polysulfone (PSf) were used as the porous supports for the composite membranes. The membranes were tested for pervaporation separation of ethylbutanoate (ETB) solutions. Sorption and desorption experiments were also performed to provide data for analysis of mass transport based on resistance-in-series model.

The composite membranes with polyether block amide (PEBA) casted on PSf (PEBA/PSf) showed superior pervaporation performance than that casted on PAN (PEBA/PAN). The analysis of transport resistances revealed that: (1) the resistances in liquid boundary of ETB were highest and, therefore, were the controlling resistances; and (2) the transport resistances in the porous supports were much lower than those in the membrane top layers.

The results on plasticizing coefficients showed that ETB plasticized the membranes (positive, k_ii) but water did not (negative, k_jj). Negative coupling coefficients (k_ij) indicated that water reduced diffusivity of ETB in the membranes and the presence of ETB enhanced water diffusion in the membranes due to positive k_ji.